Peripheral nerve and muscle disorders that produce weakness and dysfunction taken as a whole are relatively common and include amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), muscular dystrophy, and even the generalized condition of sarcopenia, or muscle deterioration in the elderly. Given a variety of discoveries in basic science, a plethora of new therapies is reaching the stage of pre-clinical and clinical testing. In order to test the efficacy of these drugs, effective, sensitive biomarkers are needed both for early disease identification and for following disease progression. One technique that has the promise of serving as an effective biomarker is electrical impedance myography (EIM). In this technique, a minute electrical current is applied to a muscle via surface electrodes and the resulting surface voltages measured. From these data, the tissue's resistance and reactance are obtained, providing structural and compositional information on the health of the underlying muscle. Several human clinical studies are demonstrating the potential power of EIM. However, in order for EIM to reach its full potential, an improved understanding of the relationship between EIM and underlying muscle pathology is still needed. In the past 4 years since the initiation of grant R01-055099, we have made substantial in-roads in that direction, exploring impedance alterations in two basic rat disease models, while also developing a set of experimental and analytic tools. In this renewal, we plan to continue to this work, expanding our studies into new and relevant models of neuromuscular disease, further refining the science and technology of EIM. Our broad hypothesis is that alterations in surface EIM data reflect the inherent electrical material properties of muscle, and that these properties are directly impacted by muscle pathology. We plan to test this by studying mouse models of four distinct neuromuscular disorders and comparing data across them and within them as the disorders progress. In addition to EIM data, behavioral, electrophysiological, and histological data will be collected. In Specific Aim 1, we will study EIM in the SOD1 G93A model of ALS as an example of a progressive motor neuron disorder. In Specific Aim 2, we will study EIM in the recently developed FVBn C/C mouse model of SMA, a disease characterized by both primary and secondary muscle degeneration. In Specific Aim 3, we will study EIM in the MDx mouse model of Duchenne muscular dystrophy, in which there are marked structural and compositional changes in muscle. And in Specific Aim 4, we will study EIM in the aged mouse model sarcopenia, a condition in which a combination of disuse and fibrotic change renders the muscle weak. As part of this work we will also apply a number of innovations, including refined measuring and analytic techniques, such as EIM assessment of contraction and anisotropic measurement and finite element modeling of electrical current flow. With the successful completion of this research, we will have greatly expanded our tools to effectively apply and interpret EIM in both pre-clinical animal studies and human clinical research.